This invention relates to a dehydrating press used for treating sludge such as sludge accumulated on the river bed, sludge in a sewage disposal plant and sludge removed in carrying out the shield driving method, particularly suitable for use for treating sludge accumulated on the river bed, and a method for treating sludge by using this dehydrating press.
Known in the art of dehydrating sludge is a method according to which sludge is dried in the sun. This method requires a large area of land and nowadays it is unrealistic to use this method in view of scarcity of land available for use for such purpose. A sludge dehydrating method currently used employs a chemical treatment of sludge prior to a dehydrating process. This method attempts to facilitate dehydration of sludge by treating sludge with a coagulant before dehydration.
Dehydration methods using a mechanical force are also used. One of these methods is a vacuum filtration which dehydrates sludge on a filtering cloth provided on the surface of a rotary drum by utilizing vacuum. A second method is a pressure filtration according to which sludge is pressed intermittently with a diaphragm in a filtering frame and then sludge is subjected to dehydration through a filtering cloth.
A third method is dehydration by a centrifugal separator. According to this mehtod, water is removed from sludge by utilizing difference in the specific gravity when the ceatrifugal separator rotated at a high speed.
A fourth method is dehydration by forming pellets. According to this method, sludge added with a high polymer coagulant and water glass is rotated in a slowly rotating drum and thereby sludge is formed into dehydrated pellets.
An ideal method for treating sludge is one which can dehydrate sludge containing a large ratio of water such as sludge accumulated on the river bed which is extremely difficult to be dehydrated and regenerate the dehydrated sludge as useful soil for land reclamation or gardening soil.
None of the above described prior art methods however has proved satisfactory from this standpoint.
The method utilizing vacuum filtration requires an apparatus of a large scale and this apparatus has not a large dehydration capacity. This method is alos disadvantageous in that clogging of the filtering cloth tends to occur in a relatively short period of time.
The method utilizing pressure filtration is superior in dehydrating capacity to the vaccum filtration. This method however is not exempt from the disadvantages that it requires an apparatus of a large scale and clogging of the filtering cloth tneds to occur in a short period of time.
The method using a centrifugal separator is not effective for dehydrating sludge on the river bed and besides the centrifugal separator is very expensive.
The method of forming sludge into pellets is a relatively simple method and disorders in the system seldom take place. The degree of dehydration attained by this method is not sufficiently high so that this method cannot be applied to all types of sludge including sludge on the river bed.
As described above, the prior art dehydration methods have various defects and a satisfactory dehydration method was not found for a long time. For this reason, in some cases sludge on the river bed is left on the river bed with a solidifying agent added therein. Unless a satisfactory dehydration method is developed for treating such type of sludge, the sludge treatment will become a serious environmental problem.
It is, therefore, an object of the present invention to provide a sludge dehydrating device and method which have eliminated the above described disadvantages of the prior art methods and can dehydrate sludge efficiently with a relatively simple structure and without causing the problem such as clogging of a filtering cloth.